Unit 5: Bio-imaging and Biological effects of Radiation

Bio-imaging Techniques

Bio-imaging (or medical imaging) is the set of techniques used to create visual representations of the interior of the body for clinical analysis and medical diagnosis.

X-ray

• What it is: A 2D "shadow" image.
• How it works: High-energy electromagnetic waves (X-rays) are passed through the body.
  • Dense tissues (like bone) are good at absorbing X-rays. Very few get through to the detector. These areas appear white.
  • Soft tissues (like muscle, fat, organs) are less dense. X-rays pass through them more easily. These areas appear grey.
  • Air (like in the lungs) does not absorb X-rays at all. These areas appear black.
• Uses: Best for looking at bones (fractures), teeth (cavities), and checking for dense tumors or pneumonia in the lungs.
• Risk: Uses ionizing radiation, so there is a small risk of cancer. The dose is very low and generally considered safe.

CT scan (Computed Tomography)

• What it is: A "3D X-ray" that creates detailed cross-sectional "slices" of the body.
• How it works: An X-ray source and its detector are mounted on a large ring (a gantry) that rotates around the patient.
• It takes hundreds of X-ray "snapshot" images from all different angles.
• A powerful computer then processes all these 2D slices and assembles them into a highly detailed 3D image. This allows doctors to see the size and location of organs, tumors, and blood vessels with great precision.
• Uses: Excellent for diagnosing tumors, internal injuries, blood clots, and complex bone fractures.
• Risk: Uses a *much higher* dose of ionizing radiation than a single X-ray, so it is used more sparingly.

Ultrasound

• What it is: A real-time image created using sound waves.
• How it works:
  1. A "transducer" (a handheld wand) sends pulses of high-frequency sound waves (ultrasound) into the body. These are sound waves above the range of human hearing (see Unit 4).
  2. These sound waves travel through the body and "echo" (reflect) back when they hit a boundary between different types of tissue (e.g., fluid and soft tissue, or soft tissue and bone).
  3. The transducer detects these returning echoes. The machine calculates the *time* it took for the echoes to return and their *strength*.
  4. From this, it builds a live, 2D image (a "sonogram") of the internal structures.
• Uses: Very safe, as it uses non-ionizing radiation (sound). Most famous for monitoring the growth of a fetus during pregnancy. Also used to look at the heart (echocardiogram), gallbladder, kidneys, and blood vessels (Doppler ultrasound).
• Risk: No known risks.

MRI imaging (Magnetic Resonance Imaging)

• What it is: A very detailed 3D imaging technique that uses a powerful magnetic field and radio waves.
• How it works:
  1. Your body is made mostly of water (H₂O). Water contains hydrogen protons. These protons act like tiny, spinning magnets.
  2. The MRI machine is a giant, powerful superconducting magnet. When you are inside, all the hydrogen protons in your body align with this strong magnetic field.
  3. The machine then sends a short pulse of a radio wave into the body. This pulse knocks the aligned protons "out of sync."
  4. When the radio pulse stops, the protons "relax" and snap back into alignment with the magnetic field. As they do, they release a tiny radio signal of their own.
  5. Different tissues (fat, muscle, brain, cartilage) have different amounts of water, so they send back different signals.
  6. Sensitive detectors pick up these signals, and a computer uses them to build an extremely detailed 3D image.
• Uses: Unmatched for imaging soft tissues. It is the best tool for looking at the brain (tumors, strokes), the spinal cord, and joint interiors (torn ligaments, cartilage).
• Risk: Uses non-ionizing radiation, so it is very safe. The main danger is from the powerful magnet (no metal implants, pacemakers, or metal objects in the room).

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Biological Effects of Radiation (General)

This topic refers specifically to the effects of ionizing radiation (X-rays, gamma rays, particle radiation).

Effect of radiation on cells

When ionizing radiation passes through a living cell, it has enough energy to "ionize" atoms—it knocks electrons out of their orbits. This creates highly reactive charged particles called free radicals.

The main "target" within the cell is the water (H₂O) molecule, which is abundant.
Radiation can split water (`H₂O`) into highly reactive free radicals like the hydroxyl radical (`·OH`), which is extremely damaging.
These free radicals can then attack and break critical molecules in the cell, most importantly DNA.

Effect in a short time (Acute Effects)

This is "Radiation Poisoning" (see Unit 4). It requires a very high dose of radiation delivered in a short time (minutes to hours).

• The radiation kills so many cells that the body's systems begin to fail.
• The most vulnerable systems are those with rapidly dividing cells:
  • Bone Marrow: Fails, leading to loss of white blood cells (infection) and platelets (bleeding).
  • Gastrointestinal (GI) Tract: Lining of the stomach and intestines is destroyed, leading to nausea, vomiting, and inability to absorb nutrients.

Low-level doses, limits

This is the radiation we are all exposed to every day ("background radiation" from soil, space, and even our own bodies) or from medical procedures (like X-rays).

• The body has mechanisms to repair most of the damage from low-level doses.
• However, the repair is not always perfect. Sometimes, the damage is "misrepaired" or not repaired at all.
• There is no *known* perfectly "safe" dose of radiation. The assumption is that *any* amount of radiation carries a small, statistical *risk* of causing long-term effects (like cancer).
• Limits: Regulatory bodies set legal dose "limits" for radiation workers and the general public. These limits are not "safe" lines; they are "acceptable risk" lines, balancing the benefits of the radiation (e.g., medical, power) with the potential harm.

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Radiation Damage to DNA

This is the most critical biological effect of radiation, as it is the primary cause of long-term problems. Radiation can damage the DNA molecule in two ways:

1. Direct Ionization of DNA

The radiation particle (e.g., a gamma ray or alpha particle) *directly* hits the DNA molecule itself.

This high-energy impact can physically break one or both strands of the DNA double helix.

• Single-Strand Break (SSB): The cell can usually repair this easily, like fixing a broken zipper.
• Double-Strand Break (DSB): This is very dangerous. The DNA molecule is broken completely in two. This is difficult for the cell to repair, and "misrepair" is common (e.g., the wrong ends are joined, or genes are deleted).

2. Indirect Ionization (via Free Radicals)

This is the most common form of damage (about 70%).

1. Radiation hits a water molecule (H₂O) in the cell.
2. It creates a highly reactive hydroxyl free radical (·OH).
3. This free radical then drifts a short distance and *chemically attacks* the nearby DNA molecule, causing a strand break.

Consequences of DNA Damage

The cell has three possible outcomes after its DNA is damaged:

1. Successful Repair: The cell's repair mechanisms (like enzymes) fix the break perfectly. The cell survives and is normal.
2. Cell Death (Apoptosis): The damage is too severe to be repaired. The cell activates a "self-destruct" program (apoptosis) and dies. This is the "good" outcome for a badly damaged cell. (This is how radiation therapy kills cancer cells).
3. Misrepair (Mutation): The cell tries to repair the break but does it wrong. It survives, but its DNA "code" is now permanently altered. This is a mutation.

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Long-term Biological Effects

These effects, known as "stochastic effects," are the result of a single cell (or a few cells) that survived with a misrepaired mutation (Consequence #3 above). The effects may not appear for many years or even generations.

Carcinogenic effects (Cancer)

• Carcinogenesis is the formation of cancer.
• If the radiation damages the specific genes that control cell division (oncogenes or tumor-suppressor genes), the cell can lose its "off switch."
• This single mutated cell may begin to divide uncontrollably, growing over years or decades into a tumor (cancer).
• This is a "stochastic" (probabilistic) effect:
  • The *probability* of getting cancer increases with the radiation dose.
  • The *severity* of the cancer, if it occurs, does not depend on the dose.

Genetic effects

• This is damage that occurs to the DNA of a sperm or egg cell (a "germ" cell).
• The person exposed to the radiation may be fine, but the mutation is passed on to their children.
• The child will have this mutation in *every cell* of their body, which can result in a heritable disease or birth defect.
• This is why protecting the reproductive organs (e.g., with a lead apron during a dental X-ray) is so important.